Java Animations: JButton, JPanel, LayoutManagers, Events, and Drawing Graphics

1. Animations with Java By Keith Lynn Field Trip #19

2. Overview • JButton • JPanel • LayoutManagers • Events • The paint method • The Graphics object • Threads • Drawing a stick figure and making it move

3. JButton • A JButton is a simple component that allows us to put a push button on a container • It is found in the javax.swing package • A JButton by default will have margins. If the label won't display, we can obtains the insets and change the left and right margin • We can attach a listener to a JButton so that clicks are detected

4. JPanel • A simple container that holds other containers • It can be used to segment a larger frame into smaller independent pieces • We can change the layout of the JPanel and it won't affect other Jpanels • We can set the preferred size of a Jpanel with the method setPreferredSize which will accept a Dimension object. e.g. setPreferredSize(new Dimension(50,20));

5. LayoutManagers • LayoutManagers define how components are layed out in a Container • There are predefined LayoutManagers: FlowLayout (the components are layed out linearly), GridLayout (the components are layed out in a grid), GridBagLayout (allows us to assign weights to components so that they can take up more than one row or column) • If we want to layout a Container ourselves, we can remove the LayoutManager by setting it to null

6. Events • Events occur when we do something to a component, e.g. clicking a button • In order to act on the event, we assign a listener to the component • Typical listeners are ActionListener for button clicks and ItemListener for selections • ActionListener and ItemListener are interfaces that require us to implement certain methods • If a listener is properly registered, then the corresponding method will be called when the event happens

7. The paint method • There is a special method inherited by components that allow us to draw on a Container • JApplet and JFrame inherit the paint method • Subclasses of JComponent, e.g. JPanel, inherit the paintComponent method which should be used instead of paint • When a Component first becomes visible or is repainted, then the paint or paintComponent methods will be called if possible

8. The Graphics object • The paint and paintComponent methods accept a parameter of type Graphics • Graphics is an abstract class. Therefore we cannot create an instance of a Graphics object. We rely on the interpreter to create a Graphics object for us • Using the Graphics objects, we can draw lines, rectangles, ovals and polygons

9. Threads • When an applet is executed, something called a thread is created • A thread is a line of execution • We should not place a loop in the applet's thread because it will not be able to respond to events • Instead we create a seperate thread

10. Threads con't • Thread implements the Runnable interface • To create a new Thread, we can • Subclass Thread and override its run method • Create a Runnable class, and then create a Thread using the class • We should never call the run method directly • In order to start a Thread we call the start method • To keep a thread from running forever, we can use a boolean to determine how long it should run

11. Drawing a Stick Figure and Making it Move • In order to make the stick figure appear to move, we draw several stick figures • We then display each of those figures in order • When we repeat this, the figure moves